1. Field of the Invention
The present invention relates to a fixing device, which is used for image forming apparatuses such as a copying machine, an optical printer, and the like, and fixing a toner image on a recording medium by heating.
2. Related Background Art
FIG. 41 is a longitudinal sectional view of a conventional thermal fixing device In a copying apparatus, a fixing device 11 is arranged at a position to which a transfer medium J as a recording medium transferred with a toner image B is fed.
The fixing device 11 comprises a fixing roller 9. The fixing roller 9 is rotatably supported by the apparatus main body, and is rotated by a driving means (not shown) in the direction of an arrow in FIG. 41. The fixing roller 9 has a cylindrical core 9a consisting of, e.g., aluminum, iron, or the like, and the surface of the core 9a is coated with a mold-releasable resin layer 9b such as PFA, PTFE, or the like. A halogen heater (heating member) 8 is arranged in the cylindrical core 9a, and is applied with a voltage from a low voltage power source 5. On the other hand, a thermistor (temperature detection element) 7 contacts the outer circumferential surface of the fixing roller 9 so as to detect the temperature of the outer circumferential surface. The thermistor 7 is connected to a control means C. The control means C controls a ON/OFF state of the halogen heater 8 on the basis of an output signal from the thermistor 7, thereby maintaining the temperature of the outer circumferential surface of the fixing roller 9 at a predetermined value. A separation pawl 18 contacts the outer circumferential surface of the fixing roller 9 so as to prevent the transfer medium J on which the toner image B has been fixed, from being wound around the fixing roller 9.
A press roller 10 is arranged under the fixing roller 9, so that its rotational shaft extends parallel to that of the press roller 9. The fixing roller 10 has a core 10a consisting of, e.g., iron, stainless steel, or the like, and an elastic member layer 10b coated on the core 10a. The elastic member layer 10b consists of silicone rubber, fluororubber, or the like with high heat resistance and good mold release properties. The press roller 10 is arranged so that its outer circumferential surface is pressed against the fixing roller 9. The press roller 10 is driven in the direction of an arrow upon rotation of the fixing roller 9.
An entrance guide member 17 is arranged at the right side of the fixing device 11 with the above arrangement. The entrance guide member 17 feeds the transfer medium J transferred with the toner image B to a nip between the fixing roller 9 and the press roller 10. When the transfer medium J is fed from the right side of the transfer device 11, as shown in FIG. 41, and is fed to the nip between the fixing roller 9 and the press roller 10, the toner image B is fixed on the transfer medium J since it is pressed against the fixing roller 9 heated to a predetermined temperature.
Since the above-mentioned conventional control does not take a drift of an input voltage to be input to the low voltage power source 5 into consideration, when the input voltage drifts, the power in watts of the halogen heater 8 drifts accordingly. The following equation expresses the relationship between the drifted input voltage V and the power W in watts of the halogen heater 8 EQU (W/W.sub.0)=(V/V.sub.0).sup.1.54 ( 1)
W.sub.0 : rated power, V.sub.0 : rated voltage
For example, a case will be examined below wherein a convey speed (to be referred to as a "process speed" hereinafter) of the transfer medium J is 49 mm/sec, the thickness of the core 9a of the fixing roller 9 is 2.5 mm, and the thermistor 7 is brought into contact with a portion which is not in direct contact with the transfer medium J so as to detect the temperature of that portion. In this case, the power in watts of the halogen heater 8 required for attaining proper fixing is about 518 W normally (when the input voltage does not drift). However, in consideration of about .+-.15% (85 V to 115 V) of the drift of the input voltage, the voltage drift occurs within a range from 85 V to 115 V, and in order to guarantee fixing characteristics in this case, a halogen heater having a rated voltage of 100 V and a rated power of 665 W must be used.
As can be understood from equation (1), the power in watts of this halogen heater 8 varies within a range from 518 W to about 825 W in response to the input voltage drift. When the power in watts of the halogen heater 8 drifts largely in this manner the elevating rate of the surface temperature on the fixing roller 9 largely varies.
FIG. 42 shows the relationship between the power in watts of the halogen heater 8 and the elevating rate of the surface temperature on the fixing roller 9. As is apparent from FIG. 42, when the power in watts of the halogen heater 8 is 518 W (input voltage=85 V), the elevating rate of the surface temperature on the fixing roller 9 is 1.6 deg/sec; when the power in watts of the halogen heater 8 is 825 W (input voltage=115 V), the elevating rate of the surface temperature on the fixing roller 9 is 3.7 deg/sec, and varies twice or more the minimum value.
Such a variation in elevating rate of the surface temperature on the fixing roller 9 according to the input voltage poses a particularly serious problem depending on an environment of the apparatus.
Assume that the power switch of the copying apparatus is OFF, and a copy operation (transfer and fixing of a toner image) must be performed immediately after the power switch is turned on in a state in which the surface temperature on the fixing roller is low (in the morning or winter). In this case, when the input voltage becomes high (e.g., 115 V), the elevating rate of the surface temperature of the fixing roller 9 is large, as described above, and hence, the control may overshoot. In this state, a safety circuit (not shown) operates, and the copying apparatus may halt. On the other hand, assume that the power in watts of the halogen heater is set to guarantee fixing characteristics when the input voltage drifts to be lowered (e.g., 85 V). If the input voltage is increased, power consumption of the halogen heater is increased, and arise the above-mentioned problem and another problem associated with an increase in power source capacity in an environment of the apparatus.
In order to solve the above-mentioned problems, a method for decreasing the power in watts of the halogen heater in only a power-ON state by controlling, e.g., the phase of an AC power source voltage is available. With this method, however, since the power in watts of the halogen heater 8 is uniformly decreased independently of the magnitude of the input voltage, for example, when the input voltage drifts by -15%, the time required until the surface temperature on the fixing roller 9 reaches a predetermined value is undesirably prolonged.
For example, assuming that the power in watts of the halogen heater 8 is decreased by 50%, the power in watts of the halogen heater 8 and the elevating rate of the surface temperature on the fixing roller 9 are respectively 412.5 W and 0.9 deg/sec when the input voltage=115 V, and are respectively 332.5 W and 0.6 deg/sec when the input voltage=100 V. Thus, the times of warm-up corresponding to these cases are respectively prolonged to 152 sec and 228 sec. In the worst case (input voltage=85 V), the power in watts of the halogen heater 8 and the elevating rate of the surface temperature on the fixing roller 9 are respectively 259 W and 0.4 deg/sec, and the time of warm-up becomes 343 sec.
The fixing characteristics of the fixing device 11 change due to various other factors in addition to the above-mentioned input voltage drift.